Page 247 - Introduction to Marine Engineering

224 Steering gear

turn using the hand control mechanism. The motor should then be
started up and, using the local hand control, operation of the steering
gear checked. Air should again be released from the pressurised
cylinders and the pumps through the appropriate vents.
During normal operation the steering gear should be made to move at
least once every two hours to ensure self lubrication of the moving parts.
No valves in the system, except bypass and air vent, should be closed.
The replenishing tank level should be regularly checked and, if low,
refilled and the source of leakage found. When not in use, that is, in
port, the steering motors should be switched off. Also the couplings of
the motors should be turned by hand to check that the pump is moving
freely. If there is any stiffness the pump should be overhauled. As with
any hydraulic system cleanliness is essential when overhauling equip-
ment and only linen cleaning cloths should be used.

Rotary vane type
With this type of steering gear a vaned rotor is securely fastened onto
the rudder stock (Figure 12.9). The rotor is able to move in a housing
which is solidly attached to the ship's structure. Chambers are formed
between the vanes on the rotor and the vanes in the housing. These
chambers will vary in size as the rotor moves and can be pressurised
since sealing strips are fitted on the moving faces. The chambers either
side of the moving vane are connected to separate pipe systems or
manifolds. Thus by supplying hydraulic fluid to all the chambers to the
left of the moving vane and drawing fluid from all the chambers on the
right, the rudder stock can be made to turn anti-clockwise. Clockwise
movement will occur if pressure and suction supplies are reversed.
Three vanes are usual and permit an angular movement of 70°: the
vanes also act as stops limiting rudder movement. The hydraulic fluid is
supplied by a variable delivery pump and control will be electrical, as
described earlier. A relief valve is fitted in the system to prevent
overpressure and allow for shock loading of the rudder.

All-electric steering

Steering gears which comprise electric control, electric power unit and
electrical transmission, are of two types, the Ward—Leonard system and
the Direct Single Motor system. Both types have a geared-down motor
drive via a pinion to a toothed quadrant.
A Ward—Leonard arrangement is shown diagrammatically in Figure
12.10. A continuously running motor-generator set has a directly
coupled exciter to provide the field current of the generator. The